Using time and/or power modulation to achieve dose gray-scaling in optical maskless lithography

ABSTRACT

In lithography applications, it is desirable to control, for example, a position or width of a printed line. An effective method of controlling these patterns and their resolution is by having as many grayscale levels as possible. The present invention comprises methods of grayscaling wherein modulation of the exposure time increases the number of grayscale levels on an object. In addition, the present invention comprises methods of grayscaling wherein modulating the power of an exposure beam provides additional grayscale levels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/630,871, filed Jul. 31, 2003 now U.S. Pat. No. 6,831,768, titled“Using Time and/or Power Modulation to Achieve Dose Gray-Scaling inOptical Maskless Lithography,” hereby incorporated by reference hereinin its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to lithography. Moreparticularly, the present invention relates to maskless lithography.

2. Related Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays (e.g., liquid crystal displays), circuit boards,various integrated circuits, and the like. A frequently used substratefor such applications is a semiconductor wafer or glass substrate. Whilethis description is written in terms of a semiconductor wafer forillustrative purposes, one skilled in the art would recognize that thisdescription also applies to other types of substrates known to thoseskilled in the art.

During lithography, a wafer, which is disposed on a wafer stage, isexposed to an image projected onto the surface of the wafer by exposureoptics located within a lithography apparatus. While exposure optics areused in the case of photolithography, a different type of exposureapparatus can be used depending on the particular application. Forexample, x-ray, ion, electron, or photon lithography each can require adifferent exposure apparatus, as is known to those skilled in the art.The particular example of photolithography is discussed here forillustrative purposes only.

The projected image produces changes in the characteristics of a layer,for example photoresist, deposited on the surface of the wafer. Thesechanges correspond to the features projected onto the wafer duringexposure. Subsequent to exposure, the layer can be etched to produce apatterned layer. The pattern corresponds to those features projectedonto the wafer during exposure. This patterned layer is then used toremove or further process exposed portions of underlying structurallayers within the wafer, such as conductive, semiconductive, orinsulative layers. This process is then repeated, together with othersteps, until the desired features have been formed on the surface, or invarious layers, of the wafer.

Step-and-scan technology works in conjunction with a projection opticssystem that has a narrow imaging slot. Rather than expose the entirewafer at one time, individual fields are scanned onto the wafer one at atime. This is accomplished by moving the wafer and reticlesimultaneously such that the imaging slot is moved across the fieldduring the scan. The wafer stage must then be asynchronously steppedbetween field exposures to allow multiple copies of the reticle patternto be exposed over the wafer surface. In this manner, the quality of theimage projected onto the wafer is maximized.

Conventional lithographic systems and methods form images on asemiconductor wafer. The system typically has a lithographic chamberthat is designed to contain an apparatus that performs the process ofimage formation on the semiconductor wafer. The chamber can be designedto have different gas mixtures and grades of vacuum depending on thewavelength of light being used. A reticle is positioned inside thechamber. A beam of light is passed from an illumination source (locatedoutside the system) through an optical system, an image outline on thereticle, and a second optical system before interacting with asemiconductor wafer.

A plurality of reticles are required to fabricate a device on thesubstrate. These reticles are becoming increasingly costly and timeconsuming to manufacture due to the feature sizes and the exactingtolerances required for small feature sizes. Also, a reticle can only beused for a certain period of time before being worn out. Further costsare routinely incurred if a reticle is not within a certain tolerance orwhen the reticle is damaged. Thus, the manufacture of wafers usingreticles is becoming increasingly, and possibly prohibitively expensive.

In order to overcome these drawbacks, maskless (e.g., direct write,digital, etc.) lithography systems have been developed. The masklesssystem replaces a reticle with a spatial light modulator (SLM) (e.g., adigital micromirror device (DMD), a liquid crystal display (LCD), or thelike). The SLM includes an array of active areas (e.g., mirrors ortransmissive areas) that are individually controlled to form a desiredpattern. These active areas are also known in the art as “pixels.” Apredetermined and previously stored algorithm based on a desiredexposure pattern is used to control the pixels. Each pixel in an SLM canvary its optical properties (e.g., amplitude/phase transmittance) in acontrollable manner so as to provide a variation of a dose delivered tothe wafer surface.

In a typical embodiment, each pixel can assume any of a limited numberof discrete states, each corresponding to a certain level of dosegray-scaling. One of the many states that the pixel can assumecorresponds to the pixel sending no light to the exposure area. Thisstate may be referred to as the dark state or the OFF state. Otherstates of the pixel correspond to the pixel being modulated so that itsends a certain fraction of the incident light to the exposure area. Inorder to be able to control the printed pattern (e.g., a position orwidth of a printed line), it is desirable to have as many grayscalelevels as possible. However, the number of grayscale levels achievableby increasing the number of discrete pixel states is limited due to atleast the following reasons.

A pattern on an SLM typically has to be updated for every laser pulse ifthe wafer scan is continuous with exposures occurring during the shortlaser pulses. If exposures are performed with a continuous light source,but the wafer is either at rest during the exposure or the smearing ofthe exposure is compensated, the pattern has to be updated at least veryfrequently. As a result, a high data transfer rate to the SLM has to bemaintained. This data transfer rate increases proportionally to thelogarithm of the number of discrete states, and the limitation on themaximum possible data transfer rate results in a limitation on thenumber of pixel states and number of grayscale levels.

Also, having a larger number of pixel states makes both the design of anSLM and the control over the states more difficult.

Therefore, what is needed is a maskless lithography system and methodthat would allow achieving a larger number of grayscale levels withoutincreasing the number of distinct pixel states.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to producing a large number ofgrayscale levels in an illumination system without increasing the numberof discrete pixel states in that system. This development providesprecision control over features printed by the system, such as theposition or the width of a line.

In one embodiment, the present invention provides a method ofgrayscaling in an illumination system including a laser, whereinchanging the time duration of the laser pulse provides additionalgrayscale levels.

In another embodiment, the present invention provides a method ofgrayscaling in an illumination system including a spatial lightmodulator (SLM), wherein altering the time during which a pixel of theSLM is activated provides additional grayscale levels.

In yet another embodiment, the present invention provides a method ofgrayscaling in an illumination system, wherein variation of the power ofan exposure beam provides additional grayscale levels.

In yet further embodiments of the present invention, variouscombinations of laser pulse duration, pixel activation timing, and laserpower are employed.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 shows a maskless lithography system having reflective SLMs,according to embodiments of the present invention.

FIG. 2 shows a maskless lithography system having transmission SLMs,according to embodiments of the present invention.

FIG. 3 shows an SLM, according to an embodiment of the presentinvention.

FIG. 4 shows more details of the SLM in FIG. 3.

FIG. 5 shows an assembly according to embodiments of the presentinvention.

FIG. 6 is a flowchart representing a first embodiment of the method ofgrayscaling, wherein changing the time duration of the laser pulseprovides additional grayscale levels, according to the presentinvention.

FIG. 7 is a flowchart representing a second embodiment of the method ofgrayscaling, wherein changing the time duration of a discrete state of apixel provides additional grayscale levels, according to the presentinvention.

FIG. 8 is a flowchart representing a third embodiment of the method ofgrayscaling, wherein changing the power of a laser pulse providesadditional grayscale levels, according to the present invention.

FIG. 9 is a flowchart representing a fourth embodiment of the method ofgrayscaling, wherein changing the power of individual beams from anillumination source provides additional grayscale levels, according tothe present invention.

FIG. 10 is a timing diagram showing an example of changing the timeduration of a discrete state of a pixel to provide additional grayscalelevels.

FIG. 11 is a block diagram representing one embodiment of projectionoptics 110.

FIG. 12 is a block diagram representing an example system in which thepresent invention may be used.

The present invention will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION OF THE INVENTION

Overview

While specific configurations and arrangements are discussed, it shouldbe understood that this is done for illustrative purposes only. A personskilled in the pertinent art will recognize that other configurationsand arrangements can be used without departing from the spirit and scopeof the present invention. It will be apparent to a person skilled in thepertinent art that this invention can also be employed in a variety ofother applications.

Embodiments of the present invention provide a method for grayscaling inan illumination system, for example in a maskless lithography machine.The system and method can be used to increase control over featuresprinted on a substrate, such as position or width of a line, whilemaintaining the number of discrete pixel states.

Maskless Lithography Systems

FIG. 1 shows a maskless lithography system 100 according to anembodiment of the present invention. System 100 includes an illuminationsource 102 that transmits light to a reflective spatial light modulator(SLM) 104 (e.g., a digital micromirror device (DMD), a reflective liquidcrystal display (LCD), or the like) via a beam splitter 106 and SLMoptics 108. SLM 104 is used to pattern the light in place of a reticlein traditional lithography systems. Patterned light reflected from SLM104 is passed through beam splitter 106 and projection optics 110 andwritten on an object 112 (e.g., a substrate, a semiconductor wafer, aglass substrate for a flat panel display, or the like).

It is to be appreciated that illumination optics can be housed withinillumination source 102, as is known in the relevant art. It is also tobe appreciated that SLM optics 108 and projection optics 110 can includeany combination of optical elements required to direct light ontodesired areas of SLM 104 and/or object 112, as is known in the relevantart.

In alternative embodiments, either one or both of illumination source102 and SLM 104 can be coupled to or have integral controllers 114 and116, respectively. Controller 114 can be used to adjust illuminationsource 102 based on feedback from system 100 or to perform calibration.Controller 116 can also be used for adjustment and/or calibration.Alternatively, controller 116 can be used for switching pixels 302 onSLM 104 between their discrete states (e.g., between one of theirgraytone states and the completely dark, or OFF state) (see FIG. 3).This can generate a pattern used to expose object 112. Controller 116can either have integral storage or be coupled to a storage element (notshown) with predetermined information and/or algorithms used to generatethe pattern or patterns.

FIG. 2 shows a maskless lithography system 200 according to a furtherembodiment of the present invention. System 200 includes an illuminationsource 202 that transmits light through a SLM 204 (e.g., a transmissiveLCD, or the like) to pattern the light. The patterned light istransmitted through projection optics 210 to write the pattern on asurface of an object 212. In this embodiment, SLM 204 is a transmissiveSLM, such as a liquid crystal display, or the like. Similar to above,either one or both of illumination source 202 and SLM 204 can be coupledto or integral with controllers 214 and 216, respectively. Controllers214 and 216 can perform similar functions as controller 114 and 116described above, and as known in the art.

Example SLMs that can be used in either of systems 100 or 200 aremanufactured by Micronic Laser Systems AB of Sweden and FraunhoferInstitute for Circuits and Systems of Germany.

Merely for convenience, reference will be made only to system 100 below.However, all concepts discussed below can also apply to system 200, aswould be known to someone skilled in the relevant arts. Otherarrangements or integration of the components and controllers of FIGS. 1and 2 will become apparent to one of ordinary skill in the art withoutdeparting from the spirit and scope of the present invention.

FIG. 3 shows details of an active area 300 of SLM 104, for example.Active area 300 includes an n×m array of pixels 302 (represented byellipsis in the figure). Pixels 302 can be mirrors on a DMD or locationson a LCD. By adjusting the physical characteristics of pixels 302, theycan be seen as being in one of their states. Digital or analog inputsignals based on a desired pattern are used to switch states of thevarious pixels 302. In some embodiments, an actual pattern being writtento object 112 can be detected and a determination can be made whetherthe pattern is outside an acceptable tolerance. If so, controller 116can be used to generate analog or digital control signals in real timeto fine-tune (e.g., calibrate, adjust, etc.) the pattern being generatedby SLM 104.

FIG. 4 shows further details of SLM 104. SLM 104 can include an inactivepackaging 400 surrounding active area 300. Also, in alternativeembodiments, a main controller 402 can be coupled to each SLM controller116 to monitor and control an array of SLMs. The dashed lines in FIG. 4represent a second SLM in an array of SLMs. More than one SLM may beadded to the array to suit the implementation design. As discussedbelow, adjacent SLMs may be offset or staggered with respect to eachother in other embodiments.

SLM Array Configurations

FIG. 5 shows an assembly 500 including a support device 502 thatreceives an array of SLMs 104. In various embodiments, as described inmore detail below, the array of SLMs 104 can have varying numbers ofcolumns, rows, SLMs per column, SLMs per row, etc., based on a number ofdesired exposures per pulse, or other implementation design criteria.The SLMs 104 can be coupled to a support device 502. Support device 502can have thermal control areas 504 (e.g., water or air channels, etc.).Support device 502 may also have areas for control logic and relatedcircuitry (e.g., see FIG. 4 showing elements 116 and element 402, whichcan be ASICs, A/D converters, D/A converters, fiber optics for streamingdata, etc.). In addition, support device 502 can have windows 506(formed within the dashed shapes) that receive SLMs 104, as is known inthe relevant art. Support device 502, SLMs 104, and all peripheralcooling or control device circuitry are referred to as an assembly.Assembly 500 can allow for a desired step size to produce the desiredstitching (e.g., connecting of adjacent elements of features on object112) and overlap for leading and trailing SLMs 104. A leading SLM is theSLM that produces the first image in a series of images on object 112during a scan, and a trailing SLM is the SLM that produces the lastimage in a series of images on object 112 during a scan. The overlap ofthe images from the leading and trailing SLMs 104 from different scansassists in removing seams that may result from adjacent, non-overlappingscans. By way of example, support device 502 can be 250 mm×250 mm or 300mm×300 mm. Support device 502 can be used for thermal management basedon being manufactured from a temperature stable material.

Support device 502 can be utilized as a mechanical backbone to ensurespacing control of SLMs 104 and for embedding the circuitry control andthe thermal control areas 504. Any electronics can be mounted on eitheror both of a back side and a front side of support device 502. Forexample, when using analog based SLMs or electronics, wires can becoupled from control or coupling systems 504 to active areas 300. Basedon being mounted on support device 502, these wires can be relativelyshorter, which reduces attenuation of analog signals compared to a casewhere the circuitry is remote from the support device 502. Also, havingshort links between the circuitry and active areas 300 can increasecommunication speed, and thus increase pattern readjustment speed inreal time.

In some embodiments, when SLM 104 or electrical devices in the circuitrywear out, assembly 500 can easily be replaced. Although it would appearthat replacing assembly 500 is more costly than just a chip on assembly500, it may in fact be more efficient to replace the entire assembly500, which can save production costs. Also, assembly 500 can berefurbished, allowing for a reduction in replacement parts if end usersare willing to use refurbished assemblies 500. Once assembly 500 isreplaced, only an overall alignment is needed before resumingfabrication.

Grayscaling Using Time Modulation

For most lithography applications, it is desirable to control, forexample, a position or width of a printed line. An effective method ofcontrolling these patterns and increasing resolution is by having asmany grayscale levels as possible.

One approach to increasing the grayscale on an object is modulating thelength of time during which the object is exposed to incoming light.FIG. 6 is a flowchart of one embodiment of the present invention inwhich the duration of an exposure is modulated. In this embodiment,illumination source 102 includes a laser (not shown). In step 602, lightfrom illumination source 102 is transmitted by SLM 104 to form a firstpattern on object 112.

Step 604 comprises changing the duration (e.g., pulse width) of a laserpulse from the laser in illumination source 102. For instance, if thelaser beam is separated into multiple parallel beams, and the relativelengths of those parallel beams are changed, the duration of the pulsewill also change. It will be obvious to one having ordinary skill in theart that any other method normally used to change the duration of alaser pulse can also be used in this embodiment.

In step 606, light from illumination source 102, this time with adifferent pulse width, is transmitted by SLM 104 to form a secondpattern on object 102. The second pattern overlaps the first pattern.The overlapping pattern creates grayscale.

Step 608 comprises repeating step 606 until the desired grayscale levelis achieved. Each time step 606 is repeated, a different range ofgrayscale levels can be produced. Combination of grayscales fromdifferent exposures gives additional grayscales.

FIG. 7 represents a second embodiment of the present invention, in whichthe duration of an exposure is modulated. Step 702 comprisesilluminating SLM 104 with light from illumination source 102. SLM 104creates a pattern in the light.

In step 704, object 112 is exposed by the patterned light reflected fromSLM 104.

Step 706 comprises creating levels of grayscale. This is achieved byswitching a portion of pixels 302 of SLM 104 from one of their states totheir secondary state earlier than other pixels 302 of SLM 104. Thesecondary state of a pixel may be a different grayscale state, in whichthe pixel sends a different fraction of the incident light to theexposure area. Alternatively, the secondary state to which the pixelswitches may be its OFF state, where the pixel sends no light to theexposure area. Step 706 is further described in FIG. 10, which is atiming diagram of an example step 706. X-axis 1002 represents increasingtime, with t representing the total time of one scan. Y-axis 1004represents the number of pixels 302 of, for example, SLM 104 that areactive at a given time. Assume at time 0 that a number N of pixels 302are active. For simplicity, also assume that the secondary state of allpixels is the OFF state. One of skill in the art will recognize thatother states may be used.

Part of the way through the scan, at time (t-β), a first portion A ofpixels 302 switch to their OFF states. Therefore, immediately after time(t-β), (N-A) pixels remain in their active states. Later, at time (t-α),a second portion B of pixels 302 switch to their OFF states. Thus,immediately after time (t-α), ((N-A)-B) pixels remain in their activestates. Finally, when the end of the scan is reached at time t, theremaining ((N-A)-B) pixels switch to their OFF states, leaving no pixelsremaining in the active state.

Grayscaling Using Power Modulation

FIG. 8 represents method 800, a third embodiment of the presentinvention. In method 800, grayscaling is produced by modulating thepower in each exposure. Method 800 is further supplemented by FIG. 11, ablock diagram representing one embodiment of projection optics 110. Inthis embodiment, projection optics 110 includes a filter 1102 andadditional optics 1104. One of skill in the art will recognize thatadditional optics 1104 may be placed in light path 1106 before, after,or on both sides of filter 1102. In addition, in further embodiments,filter 1102 may be placed anywhere in the optical path outsideprojection optics 110.

In embodiment represented in FIG. 11, projection optics 110 may alsoinclude a control system 1108 for controlling, among other things, anintensity transmission value of filter 1102. Control system 1108 may beeither manual or electronic. Control system 1108 may comprise, forexample, a switch.

In method 800, step 802 comprises passing light from illumination source102 through filter 1102 to create filtered light. Filter 1102 has afirst intensity transmission value.

In step 804, the filtered light exposes object 112 to produce a firstpattern on object 112.

In step 806, the intensity transmission value of filter 1102 is changedby, for example, control system 1108, so that filter 1102 has a secondintensity transmission value.

Step 808 comprises overlapping the first pattern with a second patternproduced by light passing through filter 1102 with the second intensitytransmission value. Exposing object 112 with a second pattern having adifferent intensity than the first pattern creates grayscale. Steps 806and 808 may be repeated to increase the number of grayscale levels onobject 112.

FIG. 9 represents method 900, a fourth embodiment of the presentinvention. In method 900, grayscaling is produced by modulatingthe-power in individual portions of a beam. Method 900 is furthersupplemented by FIG. 12, a block diagram comprising elements in a system1200 that may be used by method 900. System 1200 comprises, among otherelements, illumination source 102, a beam splitter 1202, a set offilters 1204, SLM assembly 500, and object 112. Set of filters 1204 mayinclude filters A–N, represented by ellipsis in set of filters 1204.Similarly, SLM assembly 500 may include at least the same number of SLMsas the number of filters. For example, if there are N filters in set offilters 1204, there may also be N SLMs in SLM assembly 500.

In method 900, step 902 comprises splitting a light beam fromillumination source 102 into more than one beam segments. The beamsegments will be referred to as beam segments A–N.

In step 904, beam segments A–N are passed through corresponding filtersA–N in set of filters 1204. Filters A–N modulate the power in each ofcorresponding beam segments A–N. After beam segments A–N pass throughfilters A–N, method 900 proceeds to step 906.

In step 906, beam segments A–N illuminate corresponding SLMs A–N in SLMassembly 500. Individual SLMs A–N in SLM assembly 500 then transmit theindividual beam segments A–N to object 112.

Finally, in step 908, the individual beam segments expose object 112 inan overlapping manner. Since different patterns can be created in eachof the beam segments by the individual SLMs A–N, some patterns mayexpose object 112 with a different intensity than other patterns. Thisexposure by multiple patterns with different intensities creates levelsof grayscale on object 112. The number of grayscale levels may beincreased by increasing the numbers of individual beam segments andindividual SLMs used.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A maskless lithography system, comprising: an illumination sourceconfigured to produce light beams; an object configured to be exposed bythe light beams; a variable filter disposed between the illuminationsource and the object, and configured to change intensity transmissionvalues based on a received signal; and a controller, wherein thecontroller is configured to modulate the intensity of the light beams bysending a signal to the variable filter to change intensity transmissionbetween exposures.
 2. The maskless lithography system of claim 1,further comprising between the illumination source and the object: aspatial light modulator (SLM), wherein the SLM has a plurality ofpixels, and wherein an active state of a pixel corresponds totransmitting light to the object by the pixel at a particular graylevel, and an OFF state corresponds to transmitting no light to theobject by the pixel.
 3. The maskless lithography system of claim 2,wherein the SLM is a digital micromirror device.
 4. The masklesslithography system of claim 2, wherein the SLM is a liquid crystaldisplay.
 5. The maskless lithography system of claim 1, wherein theobject is a semiconductor wafer.
 6. The maskless lithography system ofclaim 1, wherein the object is a liquid crystal display.
 7. In amaskless lithography system having a spatial light modulator (SLM), amethod of producing gray-scale on an object, the method comprising:passing a light beam through a variable filter at a first intensitytransmission value to create a light beam having a first power; exposingthe object with the light beam having the first power to produce a firstpattern; passing a light beam through the variable filter at a secondintensity transmission value to create a light beam having a secondpower; and exposing the object with the light beam having the secondpower to produce a second pattern, such that the second pattern overlapsthe first pattern and creates a range of grayscale levels on the object.8. The method of claim 7, further comprising: passing a light beamthrough the variable filter at a different intensity transmission valuefrom the first or second intensity transmission values to create a lightbeam having a different power; and exposing the object with the lightbeam having a different power to produce an additional pattern, suchthat the additional pattern overlaps the first pattern and the secondpattern.
 9. The method of claim 8, further comprising: repeating thesteps of passing a light beam through the variable filter at a differentintensity transmission value and exposing the object with the light beamhaving a different power until a desired number of grayscale levels isachieved.